config: Built-in support for multiple parameters with the same prefix

Desired output:

# ${param:join_equal}
... --foo=0xdead --foo=0xbeef --foo=0xcafe

Before:

params:
  --foo: 0xdead
  ' --foo': 0xbeef
  '  --foo': 0xcafe

After:

params:
  --foo:
    - 0xdead
    - 0xbeef
    - 0xcafe